Hop extract s have been used for many years as a flavor-additive to beer. Some advantages for using hop extracts instead of baled hops or pellets include 1) reduced freight costs, 2) improved stability and 3) a cleaner product. Hops have been extracted with such solvents as methanol, ethanol, methylene chloride, hexane and liquid carbon dioxide. Use of liquid and supercritical carbon dioxide for hop extraction gives rise to a purer extract, eliminating the problem of solvent residue.
Carbon dioxide hop extracts provide an excellent and relatively stable source of alpha acids, hop oil, and beta acids. During the boiling of the wort, the alpha acids are isomerized to the bitter, iso-alpha acids; some of the hop oil is solubilized and is believed to impart a hop aroma to the beer; the beta acids possibly provide additional bitterness.
When hop extracts are used in the kettle, the yield or conversion of the alpha acids into the iso-alpha acids in the finished beer is quite low, in the range of 20-45%. It then became generally known that the conversion of alpha acids could be improved by pre-isomerizing the alpha acids in the presence of a solid alkaline earth metal salt.
This then led to an extensive search to optimize the reaction conditions which would promote the alkaline earth metal effect on the process of isomerization. For example, in U.S. Pat. No. 5,015,491 there is reported a method for the production of an isomerized hop preparation which comprises mixing together hop extract with an alkali or alkaline earth metal compound (e.g. an oxide, hydroxide, a carbonate or other salt of Na, K, Mg or Ca), wherein the alkali or alkaline earth metal salt is used in an amount of 0.1 to 4.0 moles for each mole of total alpha-acid present in the extract, and subjecting the resulting mixtures to a temperature in the range of 80.degree.-200.degree. C. over a period of 0.1-15 minutes to effect conversation of alpha-acid to iso-alpha-acid. See also GB-A-8821420 and EP A 0 363 023.
In U.S. Pat. No. 5,013,571 there is reported a method of converting alpha-acids to hop flavors by exposing the alpha acids to an environment that simultaneously isomerizes and reduces the alpha-acids to form either tetrahydroisoalpha-acids and hexahydroisoalpha acids. The environment is defined as comprising H.sub.2 gas (atmospheric to 2000 psi) along with a hydrogenation catalyst; a pH of 8-12; an alkaline earth metal salt such as MgCl.sub.2 or CaCl.sub.2 wherein the amount added is typically about 1% to 5% relative to alpha-acids.
In U.S. Pat. No. 4,758,445 there is described a process for the production of isohumulones from a hop extract wherein the hop extract is first mixed with an aqueous solution to obtain a two-phase system. The aqueous phase is then separated and the humulones are precipitated by the addition of an alkaline earth metal salt; the alkaline earth metal humulates are then isomerized to the corresponding alkaline earth metal-isohumulate by subjecting to elevated temperatures (around 100.degree. C.) and a humidity of 90-98% over a period of 5 min to 6 hours.
In U.S. Pat. No. 4,123,561 there is reported a method of processing hops for brewing in which the hops or hops extracts are mixed with metallic oxides, such as CaO or MgO, wherein the metallic oxides are present in the amount of 1-3% by weight of the hops powder (the powder containing 9-21% alpha-acid), at a temperature of 70.degree.-90.degree. C., for a period of 25-30 minutes.
In U.S. Pat. No. 4,002,683 there is reported a process for transforming an alpha acid into an iso-alpha acid involving contacting the alpha acid with an aqueous solution of a metal ion (.e.g. magnesium chloride or calcium chloride ). More particularly , the alpha-acids are first placed in water followed by the addition of ethylene chloride and calcium chloride wherein the pH is adjusted to about 11.0-11.6. This basic solution is allowed to stand overnight while the isomerization takes place.
In U.S. Pat. No. 3,952,061 there is reported the isomerization of humulone-containing material being accelerated upon the addition of a salt of a bivalent metal, e.g. magnesium sulfate, wherein a stoichiometric amount or an excess of the salt is described as assuring complete isomerization. The other reaction conditions are described as including a pH of 4-8; temperature of 50.degree. C. and up to the boiling point of the mixture; at a time up to 15 minutes.
In U.S. Pat. No. 3,765,903 there is reported a process for the preparation of an isomerized hop extract which comprises adding to a solution of humulones or their salts a metal ion of calcium, magnesium, nickel, manganese or zinc, which metal is described as forming a metal-ion humulone complex which precipitates from the solution. This complex is then heated to provide a metal-ion-isohumulone complex which is substantially insoluble and used as a bittering agent, without regeneration to a soluble salt of isohumulone.
In British Patent No. 1,424,785 there is described a method for the isomerization of alpha-acids which comprises contacting a solution of hop extract containing alpha-acids, dissolved in a mixture of water-immiscible organic solvent, and a water-miscible solvent, with an oxide, hydroxide or carbonate of an alkaline earth metal (calcium, magnesium, zinc, strontium or barium), wherein the alkaline earth metal salt is present in equivalent molar quantity to the alpha-acids.
In British Patent No. 1,158,697 there is described a process for the isomerization of humulone to isohumulone by catalytic acceleration with MgCl.sub.2 wherein the molar ratio of salt to alpha-acid is 3:1; at a pH below 9.0; at temperatures of 50.degree. C. to the boiling point of the solutions; and for a time of about 15 minutes.
In British Patent No. 1,4 23,129 it is reported that alpha-acids can be isomerized in solution by the action of bases, e.g. oxides or hydroxides or carbonates of alkaline earth metals. It is stated therein that the isomerization employs such bases, not as catalysts, but as substantially stoichiometric reagents.
Other background art, recognizing the importance of isomerization, but not directly utilizing alkaline earth metal salts, is described in U.S. Pat. Nos. 4,302,479; 3,949,092; and 3,607,300.
Accordingly, as can be seen from the above, there is a variety of useful methods now reported to transform alpha-acids in hops or hops extract to the desired hop flavor (i.e. the iso-alpha acid) in the presence of an alkaline earth metal or earth metal salt. Nevertheless, there is a continuing effort to improve on the yield of iso-alpha acid so produced by careful adjustment of the combined reaction variables: temperature, time, pH and the stoichiometric ratio of alkaline earth metal to the starting alpha-acid compound.
It is therefore an object of this invention to develop a process of producing a pre-isomerized hop extract, or an isomerized resin extract (IRE) that is fluid, when warmed, that has fewer degradation by-products, and contains the hop oils.
It is also an object of this invention to provide a process in which the iso-alpha acids are produced in higher yields from the alpha acids by a relatively inexpensive and simple method and which uses a solubilized alkali and sub-equivalent amounts of solubilized alkaline earth metal salts.